The subject matter disclosed herein relates to monitoring and, in particular, to monitoring the distances or axial gaps between rotating and stationary parts in a turbine. Turbines are used for electrical power generation or to drive compressors and other rotary equipment. Turbines are expensive and need to be available and reliable for continuous operation. Such turbines typically include a rotor that rotates within an outer casing. The rotor includes buckets having a base and a blade. Nozzles in the casing are interspersed between the blades and provide direction to the heated vapor (steam or gas) to the blades. The vapor causes the blades to turn and, consequently, causes the rotor to turn.
Some turbines include one or more bucket stages. The bucket stages are positioned and retained axially by snap-ring type lockwires. These lockwires are typically held in the proper radial position by dowel pins staked in the turbine wheel dovetail hooks. Improper staking of a dowel pin or snapping of the lock wire can cause damage to the turbine and, consequently, a forced outage. For example, a bucket or a portion of the bucket could start moving because it is no longer constrained axially resulting in damage to downstream parts.
The most reliable bucket inspection methods utilize a borescope that enters the turbine through the casing. Such an inspection, however, requires taking the turbine out of service.